It is increasingly recognized that sedentariness and obesity are not solely individual problems but are responses to an environment that makes it difficult to be active and maintain a healthy weight. We will use data from the Black Women's Health Study (BWHS), a large prospective study of African-American women, to address the hypothesis that the neighborhood "pedestrian environment" (urban form, comprising such factors as street layout and presence of sidewalks) influences levels of physical activity and body mass index (BMI), independent of individual level factors and neighborhood socioeconomic status (SES). The study will focus on three cities (New York, Los Angeles, and Chicago) chosen because each contains a range of neighborhood types, approximately 13,000 BWHS participants live there, and detailed data on urban form are available. An expert in transportation and urban planning will compile data about the pedestrian environment at the neighborhood scale for the 3 cities, including nature and density of land use, proximity to parks, presence of sidewalks, speed and volume of traffic, and street structure. These data will be linked to the geocoded residential addresses of BWHS participants. We will also link U.S. Census data on SES to the participants' addresses. We will use factor analysis to identify a set of variables describing the neighborhood pedestrian environment that contribute complementary but not redundant information. Using multi-level longitudinal regression techniques, we will assess the influence of the pedestrian environment on levels of utilitarian walking, walking for exercise, vigorous activity, sedentariness, moderate activity, BMI, and change in weight, controlling for personal factors and neighborhood SES. We will also assess if an effect of urban form on BMI is mediated through physical activity. The proposed study, the first examination of the influence of urban form on BMI and physical activity in African-American women, is an innovative merging of techniques from the fields of transportation research and epidemiology. BWHS participants in the study cities live in a range of neighborhoods, have a wide range of physical activity levels, and have a wide range of BMI levels, including appreciable numbers of women who are obese. In view of the high prevalence of obesity and physical inactivity in African-American women, elucidation of the role of urban form on physical activity and BMI in this group is of public health importance. Societal level change may be necessary to stem the obesity epidemic, given the failure of interventions at the individual level. To make the case for policy change at the societal level, compelling evidence about the impact of the environment on physical activity and obesity is needed. The proposed study has the potential to provide such evidence and to impact policy in the domains of both transportation/urban planning and public health.